Methicillin-resistant Staphylococcus aureus (MRSA) USA300 represents a clade of genetically related strains that are a major cause of skin and soft tissue infections in the hospital as well as the community settings in otherwise healthy individuals. The annual frequency of deaths from MRSA is rapidly increasing and has surpassed those caused by HIV/AIDS. Therefore, there is an urgent need to develop new treatments against MRSA. Naturally occurring antimicrobial peptides are universal host defense molecules that have retained their potency throughout the years. To effectively exploit these interesting compounds, we have been constructing, expanding, and updating the widely used Antimicrobial Peptide Database (APD; http://aps.unmc.edu/AP/main.html). This comprehensive database facilitates naming, classification, statistical analysis, search, prediction and design of novel antimicrobials with desired properties. The APD has advanced the research and education in the antimicrobial peptide field in general and laid a solid basis for this project in particular. Based on our preliminary results, we hypothesize that most critical parameters can be extracted from the APD as a basis for designing and optimizing potent anti-MRSA peptides that cause damage on bacterial membranes, leading to bacterial death and augmenting host defense. To test our hypothesis, we have designed the following specific aims: (1) To identify the critical parameters that determine potency of anti-MRSA peptides based on the APD; (2) To elucidate the critical modulator in anti-MRSA peptides that determines mechanism of action and potential bacterial response genes; and (3) To examine the efficacy of database-designed peptides against S. aureus biofilm infection in vivo and mechanisms of immune modulation. To accomplish these aims, the PI has assembled a strong team that provides complementary expertise needed to understand host-pathogen interactions at the genetic, protein, and structural level as well as peptide-mediated immune responses in vivo using animal models. Because our database-designed compounds represent a novel anti-MRSA strategy that effectively eliminated resistant S. aureus USA300 both in vitro and in vivo, the outcome of this innovative research has great potential in providing potent anti-MRSA agents that benefit patients.